Problem: In history class, the girl to boy ratio is $3$ to $2$. If there are a total of $10$ students, how many boys are there?
Explanation: A ratio of $3$ girls to $2$ boys means that a set of $5$ students will have $3$ girls and $2$ boys. A class of $10$ students has $2$ sets of $5$ students. Because we know that there are $2$ boys in each set of $5$ students, the class must have $2$ groups of $2$ boys each. There is a total of $4$ boys in history class.